1. Field of the Invention
This invention relates to a photochemical vapor deposition apparatus.
2. Description of the Prior Art
Recently, there have been studied methods for forming a vapor-deposited film of amorphous silicon for use in the photosensitive drum of a duplicating machine or a solar cell. On the other hand, a vapor depositing method is further utilized in the formation of diverse insulating films or protective films, and a variety of vapor depositing methods have been proposed in answer to various uses. Among these methods, a photochemical vapor depositing method utilizing a photochemical reaction is being now particularly watched because of having such advantages as the film-forming rate is remarkably high and a uniform film can be formed on a portion of large area of a substrate, too.
A conventional chemical vapor depositing method utilizing a photochemical reaction comprises placing a substrate in an air-tight vessel made of material through which ultraviolet rays can be fully transmitted, feeding a photoreactive gas to flow through the vessel and applying ultraviolet rays radiated from an ultraviolet discharge lamp outside of the vessel through the wall thereof onto the substrate so that a photochemical reaction is caused to decompose the photoreactive gas and the resulting reaction product is vapor-deposited onto the substrate. In spite of having the above-mentioned remarkable advantages, this conventional photochemical deposition method which may be called an "outer discharge type", has however been found to have the defect that the reaction product is also vapor-deposited on the inner wall of the vessel, seriously impeding the transmission of ultraviolet rays.
Thus, a photochemical vapor deposition apparatus, which may be called an "inner discharge type", has been studied and developed. In the apparatus of this type, a reaction space and a discharge space are surrounded by the same vessel in an air-tight manner. The reaction space forms a passage for a photoreactive gas and in this reaction space a substrate is to be placed. In the discharge space, electric plasma discharge is generated and ultraviolet rays radiated from the plasma are directed onto the substrate to cause a photochemical reaction of decomposition of the photoreactive gas. Between the plasma and the substrate, there is no partition member that impedes the passing of the ultraviolet rays.
In such apparatus, in general, it is important to control properly the vapor pressure of mercury, which is a gas for electric discharge, in order that ultraviolet rays are generated with high efficiency from the plasma. In a case of the above-mentioned apparatus in which ultraviolet rays are applied from the outside of a vessel by an ultraviolet discharge lamp, it is feasible to control the vapor pressure of mercury relatively easily by regulating the temperature of the coolest portion of the lamp tube wall. On the contrary, in the case of the inner discharge type of vapor deposition apparatus, there are very difficult problems in controlling the vapor pressure of mercury. The reasons for the problems will be described here. In order to supply mercury vapor into the discharge space of the inner discharge type apparatus, such a method is carried out such that a mercury reservoir is provided and communicated on the way of a supply pipe for the photoreactive gas, for example, and mercury vapor is introduced into the vessel together with the flowing photoreactive gas. In this method, however, the vapor pressure of mercury in the discharge space of the vessel occasionally falls far out of its optimum value because mercury is condensed and deposited on the low temperature part of the inner wall of the pipe forming a passage therefor even though the mercury reservoir is controlled as to temperature. In addition, another method is also carried out in which a mercury reservoir is provided on the bottom of a reaction vessel so that mercury is caused to vaporize in a discharge space directly. In this method, it is required to provide a space for the mercury reservoir in the reaction vessel, and furthermore the mercury reservoir must be arranged in such a manner that it does not obstruct the application of ultraviolet rays. Also in this method, in fact, it is difficult to regulate the temperature of the mercury reservoir to a temperature different from that of the vessel and to control the vapor pressure of mercury in the discharge space.